Single proof mass based three-axis accelerometer

ABSTRACT

The present invention discloses a three-axis accelerometer. The three-axis accelerometer comprises: a substrate; at least one anchor block fixedly disposed on the substrate; a first X-axis electrode, a second X-axis electrode, a first Y-axis electrode, a second Y-axis electrode, a first Z-axis electrode and a second Z-axis electrode all fixedly disposed on the substrate; a framework suspended above the substrate and comprising a first beam column, a second beam column disposed opposite to the first beam column and at least one connecting beam connecting the first beam column and the second beam column; a proof mass suspended above the substrate; and at least one elastic connection component configured to elastically connect to the at least anchor block, the connecting beam, and the proof mass. The three-axis accelerometer can realize high-precision acceleration detection on three axes with only one proof mass, and in particular, can provide a fully differential detection signal for the Z axis, thereby greatly improving detection precision.

RELATED APPLICATION

This application claims the priority from CN Application having serialnumber 201911054180.0, filed on Oct. 31, 2019, which are incorporatedherein by reference for all purposes.

FIELD OF THE INVENTION

The present invention relates to the field of micro-electro-mechanicalsystems (MEMS), and in particular, to a single proof mass basedthree-axis accelerometer.

BACKGROUND TECHNIQUE

A MEMS accelerometer is widely used as a motion sensor. Compared withconventional accelerometers, it has advantages of small size, lightweight, low power consumption, low cost, good reliability, easyintegration, strong overload capacity, batch production and so on. TheMEMS accelerometer has become one of the main development directions ofaccelerometers, and can be widely used in fields such as aeronautics andastronautics, automobile industry, industrial automation, robotics andso on.

A conventional MEMS accelerometer usually work based on Newton'sclassical mechanics, and generally consist of three parts: a sensitiveproof mass, a fixed support, and a detection circuit. The proof mass isattached on the fixed support by means of one or more elastic elements.When an external acceleration occurs, the sensitive proof mass makes adisplacement due to inertia, and a magnitude and a direction of thedisplacement have a specific corresponding relationship with themagnitude and direction of the acceleration. The displacement causessome related physical quantities (such as capacitor, pressure,resistance, and resonant frequency) to change correspondingly.Therefore, if the changes of these physical quantities can be convertedinto easy-to-measure electrical quantities, such as voltage, current,frequency, etc., through the detection circuit, the displacement of thesensitive proof mass can be measured, thereby indirectly obtaining theacceleration to be measured. In addition, according to the measuredacceleration, a speed of the sensitive proof mass may be obtained byperforming integral calculation once, and a movement distance of thesensitive proof mass may be obtained by performing integral calculationtwice.

However, one mass is usually used to measure the acceleration of twoaxes currently. Even though one mass can sometimes be used to measurethe acceleration of three axes, the detection accuracy of the Z axis isvery low.

Therefore, it is necessary to provide an improved solution to solve theforegoing problem.

SUMMARY OF THE INVENTION

This section is for the purpose of summarizing some aspects of thepresent invention and to briefly introduce some preferred embodiments.Simplifications or omissions in this section as well as in the abstractmay be made to avoid obscuring the purpose of this section and theabstract. Such simplifications or omissions are not intended to limitthe scope of the present invention.

The present invention generally pertains to provide a three-axisaccelerometer, which can provide high-precision acceleration detectionon three axes based on one proof mass.

According to one aspect of the present invention, The three-axisaccelerometer provided according to one embodiment of the presentinvention comprises: a substrate; at least one anchor block fixedlydisposed on the substrate; a first X-axis electrode, a second X-axiselectrode, a first Y-axis electrode, a second Y-axis electrode, a firstZ-axis electrode and a second Z-axis electrode all fixedly disposed onthe substrate; a framework suspended above the substrate, and comprisinga first beam column, a second beam column disposed opposite to the firstbeam column, and at least one connecting beam connecting the first beamcolumn and the second beam column; a proof mass suspended above thesubstrate; and at least one elastic connection component configured toelastically connect to the at least anchor block, the connecting beam,and the proof mass. A third Z-axis electrode is formed on the first beamcolumn, a fourth Z-axis electrode is formed on the second beam column,the first Z-axis electrode and the third Z-axis electrode are disposedoppositely to form a first Z-axis capacitor, and the second Z-axiselectrode and the fourth Z-axis electrode are disposed oppositely toform a second Z-axis capacitor. A third X-axis electrode and a thirdY-axis electrode are formed on the proof mass, the first X-axiselectrode and the third X-axis electrode are disposed oppositely to forma first X-axis capacitor, the second X-axis electrode and the thirdX-axis electrode are disposed oppositely to form a second X-axiscapacitor, the first Y-axis electrode and the third Y-axis electrode aredisposed oppositely to form a first Y-axis capacitor, and the secondY-axis electrode and the third Y-axis electrode are disposed oppositelyto form a second Y-axis capacitor.

In one embodiment, the framework, the proof mass, the at least oneelastic connection component, and the at least one anchor block togetherform a proof mass electrode.

In one embodiment, when there is an acceleration on an X axis, the atleast one elastic connection component elastically deforms, the proofmass moves along the X axis, a gap between the first X-axis electrodeand the third X-axis electrode changes to cause change of the firstX-axis capacitor, a gap between the second X-axis electrode and thethird X-axis electrode changes to cause change of the second X-axiscapacitor, and the changes of the first X-axis capacitor and the secondX-axis capacitor are opposite, so that the acceleration on the X axis isobtained by detecting a change difference between the first X-axiscapacitor and the second X-axis capacitor. When there is an accelerationon a Y axis, the elastic connection component elastically deforms, theproof mass moves along the Y axis, a gap between the first Y-axiselectrode and the third Y-axis electrode changes to cause change of thefirst Y-axis capacitor, and a gap between the second Y-axis electrodeand the third Y-axis electrode changes to cause change of the secondY-axis capacitor, so that the acceleration on the Y axis is obtained bydetecting a change difference between the first Y-axis capacitor and thesecond Y-axis capacitor. When there is an acceleration on a Z axis, theelastic connection component elastically deforms, the proof mass movesalong the Z axis to drive the framework to rotate, a gap between thefirst Z-axis electrode and the third Z-axis electrode becomes larger orsmaller to cause the first Z-axis capacitor to become smaller or larger,and a gap between the second Z-axis electrode and the fourth Z-axiselectrode becomes smaller or larger to cause the second Z-axis capacitorto become larger or smaller, so that the acceleration on the Z axis isobtained by detecting a change difference between the first Z-axiscapacitor and the second Z-axis capacitor.

The three-axis accelerometer of the present invention can realizehigh-precision acceleration detection on three axes with only one proofmass, and in particular, can provide a fully differential detectionsignal for the Z axis, thereby greatly improving the detectionprecision.

There are many other objects, together with the foregoing attained inthe exercise of the invention in the following description and resultingin the embodiment illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will be better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is a schematic top view of a three-axis accelerometer accordingto one embodiment of the present invention;

FIG. 2 is a schematic sectional view of the three-axis accelerometeralong a section line A-A in FIG. 1;

FIG. 3 is an enlarged schematic diagram of a partial structure of thethree-axis accelerometer in FIG. 1;

FIG. 4 is a further enlarged schematic diagram of the three-axisaccelerometer in FIG. 3;

FIG. 5 is a schematic structural diagram of the three-axis accelerometeraccording to one embodiment of the present invention at a predeterminedmoment when there is an acceleration on an X axis;

FIG. 6 is a schematic structural diagram of the three-axis accelerometeraccording to one embodiment of the present invention at a predeterminedmoment when there is an acceleration on a Y axis; and

FIG. 7 is a schematic structural diagram of the three-axis accelerometeraccording to one embodiment of the present invention at a predeterminedmoment when there is an acceleration on a Z axis.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description of the invention is presented largely in termsof procedures, steps, logic blocks, processing, and other symbolicrepresentations that directly or indirectly resemble the operations ofcommunication or storage devices that may or may not be coupled tonetworks. These process descriptions and representations are typicallyused by those skilled in the art to most effectively convey thesubstance of their work to others skilled in the art.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments mutuallyexclusive of other embodiments. Further, the order of blocks in processflowcharts or diagrams representing one or more embodiments of theinvention do not inherently indicate any particular order nor imply anylimitations in the invention.

The present invention provides a three-axis accelerometer, which mayprovide high-precision acceleration detection on three axes based on oneproof mass.

FIG. 1 is a schematic top view of a three-axis accelerometer accordingto one embodiment of the present invention. FIG. 2 is a schematicsectional view of the three-axis accelerometer along a section line A-Ain FIG. 1. FIG. 3 is an enlarged schematic diagram of a partialstructure of the three-axis accelerometer in FIG. 1. FIG. 4 is a furtherenlarged schematic diagram of the three-axis accelerometer in FIG. 3.

As shown in FIG. 1 and FIG. 2, the three-axis accelerometer 100includes: a substrate 10; a first anchor block 1 a and a second anchorblock 1 b both fixedly disposed on the substrate 10; a first X-axiselectrode 3 a, a second X-axis electrode 3 b, a first Y-axis electrode 4a, a second Y-axis electrode 4 b, a first Z-axis electrode 6 a, and asecond Z-axis electrode 6 b all fixedly disposed on the substrate 10; aframework 60 suspended above the substrate 10 and comprising a firstbeam column 61, a second beam column 62 opposite to the first beamcolumn 61, a first connecting beam 63 and a second connecting beam 64connecting the first beam column and the second beam column; a proofmass 2 suspended above the substrate 10; a first elastic connectioncomponent 71 and a second elastic connection component 72. The firstelastic connection component 71 is elastically connected to the firstanchor block 1 a, the first connecting beam 63 and the proof mass 2. Thesecond elastic connection component 72 is elastically connected to thesecond anchor block 1 b, the second connecting beam 64 and the proofmass 2.

A third Z-axis electrode 5 a is formed on the first beam column 61, anda fourth Z-axis electrode 5 b is formed on the second beam column 62.The first Z-axis electrode 6 a and the third Z-axis electrode 5 a aredisposed opposite to each other to form a first Z-axis capacitor, andthe second Z-axis electrode 6 b and the fourth Z-axis electrode 5 b aredisposed opposite to each other to form a second Z-axis capacitor. Theframework 60 defines a space, and the proof mass 2 is located in theframework 60. A third X-axis electrode 3 c and a third Y-axis electrode4 c are formed on the proof mass 2. The first X-axis electrode 3 a andthe third X-axis electrode 3 c are disposed opposite to each other toform a first X-axis capacitor, and the second X-axis electrode 3 b andthe third X-axis electrode 3 c are disposed opposite to each other toform a second X-axis capacitor. The first Y-axis electrode 4 a and thethird Y-axis electrode 4 c are disposed opposite to each other to form afirst Y-axis capacitor, and the second Y-axis electrode 4 b and thethird Y-axis electrode 4 c are disposed opposite to each other to form asecond Y-axis capacitor.

In one embodiment, the framework 60, the proof mass 2, the elasticconnection components 71 and 72, and the anchor blocks 1 b and 1 atogether form a proof mass electrode. That is, electric potentials ofthese components are consistent, and these components form the sameelectrode. For example, the framework 60, the proof mass 2, the elasticconnection components 71 and 72, and the anchor blocks 1 b and 1 a mayall be formed by conductor, semiconductor materials or compositematerials so that electric potentials of these components areconsistent. In this way, the proof mass electrode may provide the sameelectric potential for the third Z-axis electrode 5 a, the fourth Z-axiselectrode 5 b, the third X-axis electrode 3 c, and the third Y-axiselectrode 4 c.

With reference to FIG. 1 to FIG. 4, each of the elastic connectioncomponents 71 and 72 includes: a connecting portion 81 connected to theanchor block 1 a or 1 b (shown in FIG. 4); a first elastic portion 53connected to one end of the connecting portion 81; first 66 connected tothe other end of the connecting portion 81; a first elastic arm 51connected to the first elastic portion 53; a second elastic arm 57connected to the second elastic portion 66; a third elastic portion 54connected between the first elastic arm 51 and the second elastic arm57; a fourth elastic portion 55 connected between the first elastic arm51 and a middle portion of the connecting beam; a fifth elastic portion67 connected between the first elastic arm 51 and one side of the proofmass 2; and a sixth elastic portion 68 connected between the secondelastic arm 57 and the other side of the proof mass 2. A point where thefourth elastic portion 55 is connected to the first elastic arm 51 and apoint where the fourth elastic portion 55 is connected to the connectingbeam is spaced by a predetermined distance D in an X axis direction(shown in FIG. 4).

As shown in FIG. 1 and FIG. 3, the fifth elastic portion 67 includes afirst elastic member 41 extending in a Y axis direction and a secondelastic member 42 extending in the X axis direction. The first elasticmember 41 is connected to the proof mass 2, and the second elasticmember 42 is connected to the first elastic arm 51. The sixth elasticportion 68 includes a first elastic member 43 extending in the Y axisdirection and a second elastic member 44 extending in the X axisdirection. The first elastic member 43 is connected to the proof mass 2,and the second elastic member 44 is connected to the second elastic arm57.

As shown in FIG. 3 and FIG. 4, each of the connecting beams 63 and 64includes a first end portion 52 connected to the first beam column 61, asecond end portion 58 connected to the second beam column 62, and amiddle portion located between the first end portion 52 and the secondend portion 58. The middle portion of the each of the connecting beams63 and 64 includes a neck portion 56 located on an inner side (shown inFIG. 4) and a supporting portion 59 located on an outer side. One sideof the supporting portion 59 is connected to the second end portion 58,and the other side is disconnected from the first end portion 52. Thereis a gap between the neck portion 56 and the supporting portion 59. Theneck portion 56 makes the middle portion of the connecting beam moreelastic, and the supporting portion 59 makes the middle portionstronger. The fourth elastic portion 55 is connected to a midpointposition of the neck portion 56.

FIG. 5 is a schematic structural diagram of the three-axis accelerometeraccording to one embodiment of the present invention at a predeterminedmoment when there is an acceleration on an X axis. When there is anacceleration on the X axis, the elastic connection components 71 and 72elastically deform, the proof mass 2 moves along the X axis, and a gapbetween the X-axis electrodes changes to cause change of the X-axiscapacitor. For example, when a gap between the first X-axis electrode 3a and the third X-axis electrode 3 c becomes larger, the first X-axiscapacitor becomes smaller, and when a gap between the second X-axiselectrode 3 b and the third X-axis electrode 3 c becomes smaller, thesecond X-axis capacitor becomes larger. In another example, when the gapbetween the first X-axis electrode 3 a and the third X-axis electrode 3c becomes smaller, the first X-axis capacitor becomes larger, and whenthe gap between the second X-axis electrode 3 b and the third X-axiselectrode 3 c becomes larger, the second X-axis capacitor becomessmaller. As a result, the acceleration on the X axis may be obtained bydetecting a change difference between the first X-axis capacitor and thesecond X-axis capacitor. It needs to be known that FIG. 5, FIG. 6 andFIG. 7 are all schematic diagrams of demonstrations of athree-dimensional model of the three-axis accelerometer. In order tofacilitate understanding, an action amplitude of the three-dimensionalmodel greatly exceeds an actual action amplitude.

FIG. 6 is a schematic structural diagram of the three-axis accelerometeraccording to one embodiment of the present invention at a predeterminedmoment when there is an acceleration on a Y axis. When there is anacceleration on the Y axis, the elastic connection components 71 and 72elastically deform, the proof mass 2 moves along the Y axis, and achange of a gap between the Y-axis electrodes causes the Y-axiscapacitor to change. For example, when a gap between the first Y-axiselectrode 4 a and the third Y-axis electrode 4 c becomes larger, thefirst Y-axis capacitor becomes smaller, and when a gap between thesecond Y-axis electrode 4 b and the third Y-axis electrode 4 c becomessmaller, the second Y-axis capacitor becomes larger. In another example,when the gap between the first Y-axis electrode 4 a and the third Y-axiselectrode 4 c becomes smaller, the first Y-axis capacitor becomeslarger, and when the gap between the second Y-axis electrode 4 b and thethird Y-axis electrode 4 c becomes larger, the second Y-axis capacitorbecomes smaller. As a result, the acceleration on the Y axis may beobtained by detecting a change difference between the first Y-axiscapacitor and the second Y-axis capacitor.

FIG. 7 is a schematic structural diagram of a three-axis accelerometeraccording to one embodiment of the present invention at a predeterminedmoment when there is an acceleration on a Z axis. When there is anacceleration on the Z axis, the elastic connection components 71 and 72elastically deform, the proof mass 2 moves along the Z axis, and theframework 60 rotates. A gap between the first Z-axis electrode and thethird Z-axis electrode becomes larger or smaller, which causes the firstZ-axis capacitor to become smaller or larger. When the gap becomeslarger, the capacitor becomes smaller, and when the gap becomes smaller,the capacitor becomes larger. A gap between the second Z-axis electrodeand the fourth Z-axis electrode becomes smaller or larger, which causesthe second Z-axis capacitor to become larger or smaller. When the firstZ-axis capacitor becomes larger, the second Z-axis capacitor becomessmaller, and when the first Z-axis capacitor becomes smaller, the secondZ-axis capacitor becomes larger. As a result, the acceleration on the Yaxis is obtained by detecting a change difference between the firstZ-axis capacitor and the second Z-axis capacitor. For example, as shownin FIG. 7, in this case, the proof mass 2 moves downward, the first beamcolumn 61 of the framework 60 rotates downward, the first Z-axiscapacitor becomes larger, the second beam column 62 rotates upward, andthe second Z-axis capacitor becomes smaller, so that the acceleration onthe Z axis is further detected by detecting a change difference betweenthe first Z-axis capacitor and the second Z-axis capacitor.

As shown in FIG. 4, the point where the first elastic arm 51 isconnected to the fourth elastic portion 55 and the midpoint of the neckportion 56 of the connecting beam are spaced by the predetermineddistance D in the X axis direction, namely, the structure isasymmetrical. When there is an acceleration in the Z axis direction, theentire proof mass 2 drives the framework 60 to rotate due to thisasymmetrical structure, which causes the first Z-axis capacitor and thesecond Z-axis capacitor to change conversely, thereby obtaining a fullydifferential signal of the Z axis and improving the detection precisionof the Z axis.

In one alternative embodiment, only one anchor block, only oneconnecting beam and only one elastic connection component are comprisedin the three-axis accelerometer, the framework does not define a spacein this embodiment, wherein the elastic connection component iselastically connected to the anchor block, the connecting beam, and theproof mass.

The foregoing descriptions are merely preferred embodiments of thepresent invention and are not intended to limit the present invention.Any modification, equivalent replacement, and improvement made withoutdeparting from the spirit and principle of the present invention shallfall within the protection scope of the present invention.

While the present invention has been described with reference tospecific embodiments, the description is illustrative of the inventionand is not to be construed as limiting the invention. Variousmodifications to the present invention can be made to the preferredembodiments by those skilled in the art without departing from the truespirit and scope of the invention as defined by the appended claim.Accordingly, the scope of the present invention is defined by theappended claims rather than the forgoing description of embodiments.

What is claimed is:
 1. A three-axis accelerometer, comprising: asubstrate; at least one anchor block fixedly disposed on the substrate;a first X-axis electrode, a second X-axis electrode, a first Y-axiselectrode, a second Y-axis electrode, a first Z-axis electrode and asecond Z-axis electrode all fixedly disposed on the substrate; aframework suspended above the substrate and comprising a first beamcolumn, a second beam column opposite to the first beam column and atleast one connecting beam connecting the first beam column and thesecond beam column; a proof mass suspended above the substrate; and atleast one elastic connection component configured to elastically connectto the at least anchor block, the connecting beam, and the proof mass;wherein a third Z-axis electrode is formed on the first beam column, afourth Z-axis electrode is formed on the second beam column, the firstZ-axis electrode and the third Z-axis electrode are disposed oppositelyto form a first Z-axis capacitor, and the second Z-axis electrode andthe fourth Z-axis electrode are disposed oppositely to form a secondZ-axis capacitor; and wherein a third X-axis electrode and a thirdY-axis electrode are formed on the proof mass, the first X-axiselectrode and the third X-axis electrode are disposed oppositely to forma first X-axis capacitor, the second X-axis electrode and the thirdX-axis electrode are disposed oppositely to form a second X-axiscapacitor, the first Y-axis electrode and the third Y-axis electrode aredisposed oppositely to form a first Y-axis capacitor, and the secondY-axis electrode and the third Y-axis electrode are disposed oppositelyto form a second Y-axis capacitor.
 2. The three-axis accelerometeraccording to claim 1, wherein the framework, the proof mass, the atleast one elastic connection component, and the at least one anchorblock together form a proof mass electrode.
 3. The three-axisaccelerometer according to claim 1, wherein: when there is anacceleration on an X axis, the at least one elastic connection componentelastically deforms, the proof mass moves along the X axis, a gapbetween the first X-axis electrode and the third X-axis electrodechanges to cause change of the first X-axis capacitor, a gap between thesecond X-axis electrode and the third X-axis electrode changes to causechange of the second X-axis capacitor, and the changes of the firstX-axis capacitor and the second X-axis capacitor are opposite, so thatthe acceleration on the X axis is obtained by detecting a changedifference between the first X-axis capacitor and the second X-axiscapacitor; when there is an acceleration on a Y axis, the elasticconnection component elastically deforms, the proof mass moves along theY axis, a gap between the first Y-axis electrode and the third Y-axiselectrode changes to cause change of the first Y-axis capacitor, and agap between the second Y-axis electrode and the third Y-axis electrodechanges to cause change of the second Y-axis capacitor, so that theacceleration on the Y axis is obtained by detecting a change differencebetween the first Y-axis capacitor and the second Y-axis capacitor; andwhen there is an acceleration on a Z axis, the elastic connectioncomponent elastically deforms, the proof mass moves along the Z axis todrive the framework to rotate, a gap between the first Z-axis electrodeand the third Z-axis electrode becomes larger or smaller to cause thefirst Z-axis capacitor to become smaller or larger, and a gap betweenthe second Z-axis electrode and the fourth Z-axis electrode becomessmaller or larger to cause the second Z-axis capacitor to become largeror smaller, so that the acceleration on the Z axis is obtained bydetecting a change difference between the first Z-axis capacitor and thesecond Z-axis capacitor.
 4. The three-axis accelerometer according toclaim 1, wherein: the at least one anchor block comprises a first anchorblock and a second anchor block spaced apart from each other; the atleast one connecting beam comprise a first connecting beam and a secondconnecting beam, two ends of each connecting beam are respectivelyconnected to the first beam column and the second beam column, theframework defines a space, and the proof mass is located in theframework; and the at least one elastic connection component comprises afirst elastic connection component and a second elastic connectioncomponent, the first elastic connection component is elasticallyconnected to the first anchor block, the first connecting beam and theproof mass, and the second elastic connection component is elasticallyconnected to the second anchor block, the second connecting beam and theproof mass.
 5. The three-axis accelerometer according to claim 1,wherein the at least one elastic connection component comprises: aconnecting portion connected to the at least one anchor block; a firstelastic portion connected to one end of the connecting portion; a secondelastic portion connected to the other end of the connecting portion; afirst elastic arm connected to the first elastic portion; a secondelastic arm connected to the second elastic portion; a third elasticportion connected between the first elastic arm and the second elasticarm; a fourth elastic portion connected between the first elastic armand a middle portion of the at least one connecting beam, a point wherethe fourth elastic portion is connected to the first elastic arm and apoint where the fourth elastic portion is connected to the connectingbeam being spaced by a predetermined distance in an X axis direction; afifth elastic portion connected between the first elastic arm and oneside of the proof mass; and a sixth elastic portion connected betweenthe second elastic arm and the other side of the proof mass.
 6. Thethree-axis accelerometer according to claim 5, wherein each of the fifthelastic portion and the sixth elastic portion comprises a first elasticmember extending in a Y axis direction and a second elastic memberextending in the X axis direction, wherein the first elastic member isconnected to the proof mass, and the second elastic member is connectedto the first elastic arm or the second elastic arm.
 7. The three-axisaccelerometer according to claim 5, wherein the connecting beamcomprises a first end portion connected to the first beam column, asecond end portion connected to the second beam column, and a middleportion located between the first end portion and the second endportion, the middle portion of the connecting beam comprises a neckportion located on an inner side and a supporting portion located on anouter side, a gap is disposed between the neck portion and thesupporting portion, one side of the supporting portion is connected tothe second end portion, and the other side is disconnected from thefirst end portion; and the fourth elastic portion is connected to amidpoint of the neck portion of the connecting beam.